The present invention relates to the communication of compressed digital signals and more particularly to the distribution of compressed cable television signals or the like within an available bandwidth.
In cable television networks, subscribers are connected to a transmission stream carrying, e.g., television programs, radio programs and associated data which originates at a headend. To generate the transmission stream, the headend receives signals from a variety of sources including, for example, broadcast stations, data sources and premium channels via satellite. The signals are combined at the headend for retransmission to subscribers over the CATV network.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast over cable television networks or by satellite to cable television affiliates and/or directly to home satellite television receivers. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have largely replaced analog phonograph records in the audio industry.
A substantial amount of digital data must be transmitted in any digital television system. This is particularly true where high definition television (HDTV) is provided. In a digital television system, a subscriber typically receives the digital data stream via a receiver/descrambler that provides video, audio and data to the subscriber. In order to most efficiently use the available radio frequency spectrum, it is advantageous to compress the digital television signals to minimize the amount of data that must transmitted.
The video portion of a television signal comprises a sequence of video images (typically "frames") that together provide a moving picture. In digital television systems, each line of a video frame is defined by a sequence of digital data bits referred to as "pixels." A large amount of data is required to define each video frame of a television signal. For example, 7.4 megabits of data is required to provide one video frame at NTSC (National Television System Committee) resolution. This assumes a 640 pixel by 480 line display is used with 8 bits of intensity value for each of the primary colors red, green and blue. High definition television requires substantially more data to provide each video frame. In order to manage this amount of data, particularly for HDTV applications, the data must be compressed.
Video compression techniques that enable the efficient transmission of digital video signals over conventional communication channels are well known. Examples can be found, for example, in Krause, et al. U.S. Pat. Nos. 5,057,916; 5,068,724; 5,091,782; and 5,093,720. Such techniques use compression algorithms that take advantage of the correlation among adjacent pixels in order to derive a more efficient representation of the most important information in a video signal. The most powerful compression systems not only take advantage of spacial correlation, but can also utilize similarities among adjacent frames to further compact the data.
Motion compensation is one of the most effective tools for accounting for and reducing the amount of temporal redundancy in sequential video frames. One of the most-effective ways to apply motion compensation in video compression applications is by differential encoding. In this case, the differences between two consecutive images (e.g., "frames") are attributed to simple movements. The encoder estimates or quantifies these movements by observing the two frames and sends the results to a decoder. The decoder uses the received information to transform the first frame, which is known, in a way that it can used to effectively predict the appearance of the second frame, which is unknown. The encoder reproduces the same prediction frame as the decoder, and then sends just the difference between the prediction frame and the actual frame. In this way, the amount of information needed to represent the image sequence can be significantly reduced, particularly when the motion estimation model closely resembles the frame to frame changes that actually occur. This technique can result in a significant reduction in the amount of data that needs to be transmitted once simple coding algorithms are applied to the prediction error signal.
In order to further compress the digital data, the prediction error signal can be transform coded. In transform coding, the video signal is subjected to an invertible transform, then quantized and variable length encoded. The purpose of the transformation is to convert statistically dependent picture elements into a set of statistically independent coefficients. In practice, one of the separable fast transforms in the class of unitary transforms is used, for example, cosine, Fourier or Hadamard. The most commonly used transform is the discrete cosine transform (DCT). The DCT is used in the MPEG and the DigiCipher.RTM. digital television standards.
In existing CATV distribution networks, television signals are transmitted over satellite for continental distribution and then retransmitted from different ground-based cites over cable or by terrestrial broadcasts for local distribution. Very high quality is typically maintained at a relatively high cost per receiver in the satellite link. This provides those applications that require high quality with a signal level they need. For the local distribution links to individual homes, cost generally has higher priority than signal quality. In order to reduce distribution costs, picture quality may be allowed to be degraded by ghosts, interference, cross modulation and the like. For those few sites that require higher quality signals, such as local broadcast center to CATV headends, the downlinked signals can be delivered via alternate means, such as dedicated fiber optic links, to maintain the high quality necessary.
For distribution of digital video signals, the transmission quality above a certain threshold has little effect on picture quality. As noted above, DCT based compression algorithms are commonly employed in the encoding of digital video signals. The extent of the compression will affect video picture quality. Therefore, the highest quality links available use very low loss or even lossless compression with high relative data rates and therefore a high link cost. Lower cost links can be provided by using compression with higher loss, resulting in lower data rates. As the loss resulting from higher compression increases, the data rate, distribution cost and picture quality will decrease.
It would be advantageous to provide a scheme that provides a high quality signal with relatively low loss compression over the primarily satellite link, with a lower quality, more highly compressed signal that can be distributed at lower cost for local distribution purposes. It would be further advantageous to provide such a scheme that requires only a minimal amount of compression related components at the redistribution sites which receive the high quality satellite signals and redistribute them locally at a higher compression level.
The present invention provides an apparatus and method that achieves the aforementioned and other advantages by only partially decompressing a received satellite signal and then recompressing the signal at a higher compression level for distribution at a lower data rate.